For example, in electronic devices represented by semiconductor devices, micromachines and so on, there may be the case where a fine conductor-filled structure, a joint structure or a functional structure each having a high aspect ratio in the inside thereof must be formed. In such case, there are known technologies for realizing a conductor-filled structure, a joint structure, a functional structure, etc. by filling a previously chosen filler in a fine space. However, it is extremely difficult to thoroughly fill a filler in a fine space having a high aspect ratio to an extent of its bottom without forming an air gap or deformation after hardening.
For example, when the case of a wafer treatment to be used for the manufacture of a semiconductor device is taken as an example, a number of fine spaces (holes) for the purpose of forming an electrode or the like are provided in a wafer, and the fine spaces have a very small hole size of, for example, not more than several ten Am. Moreover, there is frequently found the case where the thickness of the wafer is considerably thick relative to such a fine space having a fine hole size, and the aspect ratio of the fine space is 3 or more. In order to form an electrode, a conductive material must be surely filled in such a fine space having a high aspect ratio to an extent that it reaches its bottom. Accordingly, a high-degree filling technology is naturally required.
As the technology for forming an electrode, there is also known a technology of using a conductive paste composed of a mixture of a conductive metal component and an organic binder. However, a metallurgical technology using a molten metal material which is excellent in conductivity, low in loss and excellent in high-frequency characteristics attracts attention. Such a technology is disclosed in, for example, JP-A-2002-237468 and JP-A-2002-368082.
First of all, JP-A-2002-237468 discloses a technology for filling a metal in a fine space (through hole) by a molten metal backfilling process. The molten metal backfilling process as referred to herein is a process in which an atmosphere where an object (wafer) is placed is subjected to pressure reduction; the object is subsequently inserted into a molten metal while keeping the reduced pressure state; the ambient gas pressure of the molten metal is subsequently raised, thereby filling the molten metal in the space due to a difference in the ambient gas pressure before and after the insertion of a metal; and the object is subsequently lifted up from a molten metal tank and cooled in the air.
However, this molten metal backfilling process involves the following problems.    (a) When the object is lifted up from the molten metal tank and cooled, the metal surface becomes hollow in a concave state to a position which is lower than the surface of the object. For that reason, there is a concern that electrical conduction with the outside becomes incomplete.    (b) In order to solve the foregoing problem, a molten metal must be again supplied for the purpose of filling the concavity. Moreover, in order to fill the concavity, it is necessary to protrude the surface of the supplied metal higher than the surface of the object. Accordingly, a step of making the surface of the metal conform with the surface of the object, for example, a CMP (chemical mechanical polishing) step, is necessary. Such becomes a factor to cause complication of the steps and a reduction of the yield following this and so on.    (c) A further serious problem is the fact that nevertheless the foregoing complicated steps are necessary, an air gap in which the molten metal is insufficiently filled or the like is formed in the fine space, especially its bottom.
Next, JP-A-2002-368082 discloses a differential pressure filling system. In this differential pressure filling system, after disposing an object having a fine space formed therein and a metal sheet within a vacuum chamber, the pressure within the vacuum chamber is reduced, the metal sheet is melted by a heating unit, and the pressure within the vacuum chamber is subsequently raised with an inter gas to atmospheric pressure or higher. As a result, the molten metal is sucked in vacuo into the fine space. Subsequently, the vacuum chamber is opened, and the metal in a molten state, which remains on the surface of a specimen, is removed, and residue is then cooled at room temperature in the air.
According to JP-A-2002-368082, it is described that since the heat capacity of the molten metal is lower than that of the molten metal backfilling process (see JP-A-2002-237468), and therefore, there are brought such effects that warpage or cracking is not formed in a specimen; that a surplus metal can be suppressed to the minimum; and that it is possible to devise the realization of a reduction in costs.
However, according to the differential pressure filling system disclosed in JP-A-2002-368082, the molten metal cannot be completely filled to the bottom of the fine space, and an air gap is formed in the inside of the fine space.
Also, since the metal in a molten state, which remains on the surface of a specimen, is removed, a part (upper end side) of the molten metal filled in the fine gap is also scraped away in that step. For that reason, the problem on the concavity still exists.
Actually, what neither wafer to be manufactured by the differential pressure filling system nor device using the same has been provided on the market is an evidence that the foregoing problem cannot be solved.
Technical difficulty generated during thoroughly filling a molten metal in a fine space is not always problematic limitedly to the case of a wafer treatment for use in semiconductor device. This can also become problematic in other electronic devices, micromachines and the like.